This invention relates to a method of improving the usefulness and integrity of motion picture films, especially intermediate film that is used in the production of motion picture print films. In particular, the invention relates to use of an aqueous film-forming polymer dispersion that is applied to the base side of a scratched motion picture film, which upon drying will minimize the appearance and impact of the scratch(s). Motion picture films treated in accordance with the invention can thus be salvaged for subsequent use.
Motion picture photographic films used in producing a release print (the film projected in movie theaters) include camera origination film, intermediate film, and the release print film. Current practice for most color motion picture production involves the use of at least four photographic steps. The first step is the recording of the scene onto a camera negative photographic film. While the original negative (typically after editing) may be printed directly onto a negative working print film in a second step to produce a direct release print, most motion picture productions use an additional two intermediate steps. Typically, the original camera negative film is printed onto a negative working intermediate film, such as Eastman Color Intermediate Film, yielding a master positive. The master positive is subsequently printed again onto an intermediate film providing a duplicate negative. Finally, the duplicate negative is printed onto a print film forming the release print.
In practice, several duplicate negative copies are produced from the master positive, and each of the duplicate negatives may then be used to make hundreds of print film copies. This multistep process helps save the integrity of the valuable original camera negative film in preparing multiple release prints. In certain situations, usually involving special effects, intermediate film may be used an additional two or more times in preparing the final duplicate negatives to be used in printing the release prints. In this case, the first duplicate negative is used to print onto intermediate film to produce a second master positive, which is in turn used to produce a second duplicate negative. The second duplicate negative may be then used for printing the release prints.
Motion picture films typically comprise one or more light sensitive silver halide emulsion imaging layers coated on one side of a film base. The occurrence of scratches on the side of the film opposite to the imaging layers (i.e., the xe2x80x9cbackxe2x80x9d side or xe2x80x9cbasexe2x80x9d side of the film) is generally more common because greater effort is taken to avoid scratching the inherently softer imaging layer side in all steps of film handling, processing, printing, and/or projecting. The presence of such scratches on original or duplicate films which are being printed can result in images of the scratch marks themselves being reproduced on the printed film. The impact of such reproduced scratch mark images, as well as actual scratch marks which may be formed in a print film, on the projected print image is of course undesirable. Accordingly, once an intermediate film obtains too many scratches to produce acceptable prints therefrom, another intermediate film needs to be prepared. Considerable time and expense during production of print films can be eliminated if minor damage to the base side of an intermediate film can be xe2x80x9chealedxe2x80x9d rather than generating a new film. Additionally, if scratches formed on the base side of a print film, visible upon projection, can be minimized or eliminated by xe2x80x9chealingxe2x80x9d, the quality and duration of use of a print film can also be improved.
The photographic film supports materials used in motion picture intermediate and print film elements typically are synthetic high molecular weight polymeric materials. These support materials may be comprised of various polymeric films, but polyester and triacetate film supports, which are well known in the art, are preferred.
One method of scratch healing which has been employed for older, acetate-based photographic films consisted of transporting the scratched film over a rotating glass wheel immersed in an organic solvent mixture containing esters, ketones, and/or chlorinated hydrocarbons. This solvent blend sufficiently softened the film support so that it could be effectively burnished by the glass wheel, thereby eliminating all but the most severe scratches. Problems with this approach include the use of toxic and flammable organic solvents plus the fact that the approach will not work with newer, less soluble polyester-based films. Although polyester film base is more resistant to scratches and abrasion than cellulose triacetate base films, scratches can still occur which could render an intermediate film useless or dramatically compromise the quality or life expectancy of a print film.
Other approaches of suppressing the impact of scratches on photographic film base have involved alterations to the method of projection. A scratch suppressing illumination system for photographic printing is described in U.S. Pat. No. 4,427,283. This approach, however, is a costly way to compensate for the efect rather than eliminate its presence.
There are numerous references for providing abrasion and scratch resistant protective overcoats for photographic elements, but they primarily rely on a thermal or radiation-cured, cross-linkable coating to be applied prior to extensive handling. Their purpose is primarily to prevent scratching rather than heal already scratched photographic film. These coatings generally tend to be expensive and in some cases employ flammable and toxic monomers and/or coating solvents. Applications utilizing such protective overcoats are described, e.g., in U.S. Pat. Nos. 3,019,131, 3,081,192, 3,097,106, 5,401,541, and 5,633,049. While such cross-linked overcoats provide effective scratch and abrasion resistance, their use requires special additional processing steps following application of the coating and the uneconomical position of treating all film, whether it is scratched or not.
Simpler, less toxic and non-flammable overcoating alternatives are described, e.g., in U.S. Pat. Nos. 5,266,455, 6,268,101, and 6,303,281. These aqueous coating formulations eliminate the issue of flammability and they are relatively non-toxic. However, they rely on either elevated temperature curing, or incorporation of matte particles to provide suitable scratch resistance. Again, they do not address the issue of rendering existing scratches invisible, but rather the prevention of scratches in the first place.
Motion picture films with polyester film supports have recently been introduced which employ a process-surviving antistat backing layer and a protective topcoat on the base side (non image layer side) of the film, where such layers are applied as part of the film manufacturing process. The primary functions of the protective overcoat or barrier layer are to prevent the antistatic layer from interacting with components of the processing solutions, and to provide abrasion resistance. Protective topcoats that may be applied over the antistatic layer can include essentially any known polymeric binder. Useful hydrophobic polymers that may be effectively employed in the protective topcoat include polyurethanes, polyesters, polyamides, polycarbonates, cellulose esters, acrylic polymers, styrenic polymers, and the like. U.S. Pat. No. 5,679,505, e.g., describes the use of aliphatic polyurethanes as preferred polymeric binders for use in motion picture film back side topcoats. While the use of such materials has improved the overall performance of the motion picture film elements, they do not address the problem of healing scratches which may nevertheless be formed after manufacture of the film elements.
European Patent 678,784 A1 is directed towards a method and device for the rejuvenating of a polyester film base, and describes coating and drying of an aqueous film-forming polymer dispersion onto the base side of a scratched polyester film. While various broad classes of polymers are mentioned, including polyurethanes, copolyesters containing recurring ester groups derived from alkylene diols and a mixture of aromatic dicarboxylic acids and an aromatic sulphodicarboxylic acid, whose sulpho group is in salt form, are described as the preferred rejuvenation, or scratch healing polymer. However, it has been found that the specifically described rejuvenation coating has several deficiencies including: blocking resistance, flexibility, solvent resistance, abrasion resistance, and drying time. Accordingly, it would be desirable to provide a rejuvenation process which did not have such deficiencies.
In accordance with the invention, a process is described for healing scratches on the base side of an imaged motion picture film element comprising a polymeric film base having one or more image layers on one side thereof, the process comprising applying an aqueous coating formulation comprised of an aqueous dispersion of a polyurethane to the scratched base side of the motion picture film element, and drying the applied coating formulation to form a polyurethane layer, wherein the polyurethane comprises an aliphatic polyurethane which has a Young""s modulus at 100% elongation to break of less than 4000 psi and a Sward hardness value of at least 60. Scratch healing formulations employed in accordance with the invention provide coated layers with superior resistance to blocking, cleaning solvents, and abrasion, and which are more flexible, dry faster, and are more effective in hiding minor scratches than that disclosed in the prior art.
This invention relates to a scratch healing or rejuvenation formulation that can easily be applied to photographic motion picture films after manufacture of such films. Films which may be treated in accordance with the invention comprise a polymeric film base and one or more image layers on one side thereof. While in most instances the image layers of a motion picture film will be exposed and developed silver halide photographic emulsion layers, the invention will be equally applicable to healing scratches on the base side of motion picture films having image layers formed by other, less conventional techniques, e.g., such as imbibition printing.
The process of the invention is particularly applicable to curing of scratches on the back side of motion picture intermediate and print film elements, where the use of a scratch healing layer in accordance with the invention prevents transfer of scratch images from an intermediate negative to a release print during the print film printing operation, or projection of scratches on a print film onto a screen during film viewing. Without treatment, intermediate films that contain scratches may transfer the image of these defects onto the print film during subsequent printing operations. Similarly, print film containing such scratches exhibit poor viewing quality when the print film image is projected onto a motion picture screen unless treated.
The photographic film supports materials used in the motion picture intermediate and print film elements which may be treated in accordance with this invention typically are synthetic high molecular weight polymeric materials. These support materials may be comprised of various polymeric films, but polyester and triacetate film supports, which are well known in the art, are preferred. While the invention may be used for films employing either such type of support, the invention is particularly suitable for the treatment of films employing polyester film supports, and in particular polyethylene terephthalate, as prior art scratch healing processes designed for use with motion picture films having acetate supports are typically not effective for polyesters.
Motion picture films employing polyester support members typically employ an antistat layer and a protective topcoat on the base side (non image side) of the film. The primary function of the protective overcoat or barrier layer is to prevent the antistatic layer from interacting with components of the processing solutions. Protective topcoats that may be applied over the antistatic layer can include essentially any known polymeric binder. Useful hydrophobic polymers that may be effectively employed in the protective topcoat include polyurethanes, polyesters, polyamides, polycarbonates, cellulose esters, acrylic polymers, styrenic polymers, and the like. Particularly preferred polymeric binders for use in the topcoat include aliphatic polyurethanes such as those described in U.S. Pat. No. 5,679,505, which is incorporated herein by reference.
Ideally, any layer subsequently applied over the protective topcoat which is coated during film manufacturing, such as a scratch healing formulation applied in accordance with the invention, should have physical properties at least as good as those of the protective topcoat layer. These properties include optical clarity with no induced color, adequate scratch and abrasion resistance, resistance to blocking in roll form and resistance to commonly employed cleaning solvents, plus an acceptable degree of curl, flexibility, and friction. In addition, it must form a uniform coated layer, dry in a reasonable time and possess adequate adhesion to the underlying layers.
Scratch healing formulations employed in accordance with the invention comprise an aqueous polyurethane dispersion, which is film forming at room temperature, the polyurethane having a combination of a relatively low Young""s modulus measured at 100% elongation to break (i.e., less than 4000 psi) coupled with a high degree of hardness (i.e., Sward hardness value of at least 60, where Sward hardness value is a percentage hardness rating relative to plate glass). Aliphatic polyurethanes generally provide excellent thermal and LW stability and freedom from yellowing, properties which are desirable for use in the present invention, and aliphatic polyurethanes having the described combination of modulus and hardness produce a dried film having a superior combination of flexibility and abrasion resistance.
Preparation of aqueous polyurethane dispersions is well-known in the art, and typically involves chain extending an aqueous dispersion of a prepolymer containing terminal isocyanate groups by reaction with a diamine or diol. The prepolymer is prepared by reacting a polyester, polyether, polycarbonate, or polyaciylate having terminal hydroxyl groups with excess polyfunctional isocyanate. The resulting product is then treated with a compound that has functional groups that are reactive with an isocyanate, for example, hydroxyl groups, and a group that is capable of forming an anion, typically a carboxylic acid group. The anionic groups are then neutralized with a tertiary amine to form the aqueous prepolymer dispersion. It is an advantage of the invention that crosslinking is generally not required to provide good performance in the scratch healing coated layers. If desired, however, the polyurethanes may be crosslinked by addition of a crosslinking agent that reacts with functional groups present in the polyurethane, for example, carboxyl groups. Suitable crosslinking agents include aziridines, carbodiimides, epoxies, and the like.
In preferred embodiments, the polyurethane is selected so as to have an index of refraction closely matching that of the film support or backing, and a surface tension such that dilute aqueous dispersions can be easily coated on existing motion picture film backings without repellency or the need for additional coating aids. For optimized coating performance, the concentration of the polyurethane in the aqueous scratch healing coating formulation is preferably from 5 to 35 wt %, more preferably from 20 to 30 wt %, and the viscosity of the coating formulation is preferably from 10 to 800 cps, more preferably from 10 to 30 cps. For optimized scratch healing performance, the dried coverage of the applied polyurethane layer is preferably from 2000 to 7000 mg/m2, more preferably from 3000 to 4000 mg/m2. For optimized drying performance, the applied coating should be able to be dried to a tack-free condition in less than 100 seconds, particularly at drying temperatures of 40xc2x0 C. or lower.
To optimize film handling performance after application of the scratch healing layer, the polyurethane coating applied in accordance with the invention is preferably resistant to blocking against the outermost layer on the image layer side of the motion picture film element when such applied coating and such outermost layer are in contact at temperatures up to 60xc2x0 C. and relative humidity up to 80%, and the applied coating further preferably provides a coefficient of friction of from 0.10 to 0.40. While it is preferred that the polyurethane itself is selected to provide such properties alone, various additives such as lubricants and matting agents may be included in the coating formulation if desired to further enhance such properties.
Methods of cleaning motion picture films include total immersion in select organic solvents. The polyurethane of scratch healing formulations employed in accordance with the invention preferably are substantially insoluble in such cleaning solvents, or otherwise exhibit sufficient solvent resistance to prevent dissolution in the cleaning solvent bath or softening and either xe2x80x9cgumming upxe2x80x9d the transport rollers or blocking within the roll once the film is wound up after treatment, to allow the dried coating to be cleaned using any of such commonly employed motion picture film cleaning solvents without the need for cross-linking. Such cleaning solvents, which may be used in various parts of the world, include 1,1,1 trichloroethane, tetrachloroethylene, isopropyl alcohol, isobutyl benzene, and other hydrocarbon solvents. Until recently, the principle cleaning solvent used by the industry has been 1,1,1 trichloroethane (a.k.a. methyl chloroform). However, this compound has been identified as an xe2x80x9cozone depletorxe2x80x9d and its use is being phased out. Tetrachloroethylene (a.k.a perchloroethylene), which is still used as a Wet-Gate printing solvent, has been used to clean film as well, but it is very slow to dry which necessitates reduction in speed of the cleaning equipment. Isopropyl alcohol is also being used as a film cleaning solvent. Its major drawback, to the industry, is a flashpoint of 12xc2x0 C. Isobutyl benzene has been used to some extent as a film cleaning solvent, but it has a strong odor, poses a moderate fire risk, and is toxic in high concentrations. ACTREL 1064-L, manufactured by the Exxon Chemical Co. is a proprietary hydrocarbon solvent and, as such, has all the flammability concerns associated with that class of solvent although it is being used as a replacement for methyl chloroform.
The following examples are intended to illustrate the present invention more practically but not to limit it in scope in any way.